On the approximation of the similarity-transformed Hamiltonian in single-reference and multireference coupled cluster theory

► Approximation of the similarity-transformed Hamiltonian in coupled cluster theory. ► Recursive approximation incorrectly treats some of the fourth-order energy terms. ► A modified scheme correct up to fourth order is comparable in accuracy to CCSD. ► The recursive approximation degrades the quality of the Mukherjee's MRCC method.